Technologies for realizing high definition of cathode-ray tubes of projection type and direct view type are essential for implementing the future high definition television (hereinafter, referred to as "HDTV") broadcasting. Also, there have been strong demands toward improvement in characteristics of cathode-ray tubes used for televisions or video displays in the existing broadcasting. In particular, a projection type cathode-ray tube used as a large-sized video display needs to be improved in resolution and to be enlarged in angle of visibility. The enhancement in both definition and resolution, however, reduces brightness of a screen of a cathode-ray tube, and therefore, it must be accompanied by improvement in luminous efficiency of a phosphor used in the cathode-ray tube. Of course, the enhancement in both high definition and high resolution has come to be required for luminous type displays other than a cathode-ray tube, for example, an electroluminescence element and a vacuum fluorescent display.
Related art cathode-ray tubes of a projecting type have been described in "Singaku Technical Report, CPM93-32(1993), pages 1-6" and "Journal of Luminescence, 48/49 (1991), pages 43-48".
Improvement in brightness of a projection type cathode-ray tube has been mainly performed by enlarging the particle size of phosphor particles and increasing the thickness of a phosphor film. The increased thickness of a phosphor film, however, causes such a problem as shown in FIG. 9. That is, for a thick phosphor film, the diameter of an electron beam (luminous spot diameter) is apparently spread by the scattering of light due to the film structure shown in FIG. 9, resulting in the degraded resolution. In FIG. 9, a phosphor film 72 (thickness indicated by numeral reference 94: 50 .mu.m) is formed of phosphor particles having diameters of 10-13 .mu.m on a face plate 71, and an Al reflection film 91 is provided on the phosphor film 72. In the case where an electron beam (diameter indicated by reference numeral 92: 110 .mu.m) impinges on the phosphor film 72 through the Al reflection film 91 and ejects from an opposing surface of the face plate 71 to the phosphor film 72, a profile 95 of a luminous spot of the electron beam becomes thicker than the incident electron beam as shown in FIG. 9. Concretely, the luminous spot diameter 93 becomes 180 .mu.m, which is 70 .mu.m thicker than the incident electron beam diameter 92 (110 .mu.m). The beam increment (70 .mu.m) indicated by reference numeral 96 is produced depending on scattering of light in the phosphor film 72 and luminous saturation of the phosphor. In FIG. 9, reference numeral 97 indicates a locus of fluorescence due to scattering and reflection of light in the phosphor film 72.
The reason why the above technique of increasing the thickness of a phosphor film, which has a problem in terms of degradation of resolution, is applied to a phosphor film of a cathode-ray tube, is that the substantial efficiency of the phosphor is reduced by increasing the density of an excitation current of the cathode ray tube. Specifically, the gradient (current coefficient) of a relationship of a luminous intensity (logarithmic scale) to an excitation current intensity (logarithmic scale) of a phosphor is deviated from the linearity, and the increase in luminous intensity becomes blunt (that is, brightness is saturated). Such a reduction in current coefficient is one of important problems of the existing projecting type cathode-ray tube. In particular, a large problem resides in reduction in current coefficient of a blue component of a blue phosphor (ZnS: Ag) in a high irradiation current range, as shown in FIG. 8. As for the blue phosphor, the spot diameter due to light scattering is significantly degraded because the particle size of the phosphor particles is 10 .mu.m or more and the thickness of the phosphor film is 50 .mu.m or more. To obtain a cathode-ray tube with a high definition and a high brightness for meeting the future requirements for high definition, there must be realized a phosphor reduced in particle size and a phosphor film thinned in thickness by solving the essential problem, that is, reduction in current coefficient.
Such a problem in characteristics is common to a green phosphor (ZnS: Cu) of a direct view type cathode ray tube. In the direct view type cathode-ray tube, an excitation density per unit area of a phosphor is also increased accompanied by the enlarged size of a screen and the enhanced definition of the cathode-ray tube. The enlarged size of the screen broadens a width of a line along which an electron beam is scanned, so that a residence time of the electron beam is shortened at each luminous point, with a result that a substantial exciting time is shortened, that is, the brightness is reduced. For the cathode-ray tube having a large-sized screen, the amount of an excitation current is increased for compensating for reduction in brightness, to strengthen the excitation current density of the phosphor. One of attempts to improve brightness is to enlarge the particle size of phosphor particles like a projecting type cathode-ray tube; however, the attempt is disadvantageous in reducing the resolution or increasing such a feeling that the screen is roughened thereby reducing the uniformity of the screen. In addition, the increased particle size of phosphor particles improves the quality of fine crystals of a powder by
i) improving the crystal quality of particles of a powdered phosphor, PA1 ii) dispersing the luminescence center, and PA1 iii) reducing the influence of surface defects due to reduction in specific surface area, to thereby improve the brightness of the phosphor screen.
As can be seen from the above description, the improvement in current coefficient which is an essential problem, is required for further enhancing brightness.
Accordingly, the basic cause, which is common to reduction in resolution and brightness, lies in reduction in a current coefficient .gamma., and therefore, it becomes apparent that the increase in such a current coefficient .gamma. is most effective for improving the brightness and resolution of a cathode-ray tube.
The current coefficient .gamma. is a non-dimensional number, and is given by an index .gamma. of a relationship that a brightness B of a phosphor screen is proportional to the .gamma.-th power of an irradiation current density Ik by irradiation of an electron beam. The relationship is given by the following equation: EQU B.varies.Ik .gamma.
In the above-described related art cathode-ray tube, the improvement in brightness has been performed by enlargement of particles size of phosphor particles without improvement in current coefficient .gamma., and an examination has been not sufficiently made to further improve brightness without reduction in resolution. In view of the foregoing, there have been expected a phosphor material capable of realizing a high resolution by reducing a current coefficient, and a cathode-ray tube and a spontaneously emissive type display using such a phosphor material.